Final figures from the medals standings at the world championships, which wrapped up Sunday, offered evidence that track and field’s attempt to crack down on a global doping problem could be making a direct impact on the results themselves.The United States won 30 medals, while athletes from four countries that have been under the doping microscope — Kenya (11), Russia (6), Ethiopia (5) and Jamaica (4) — combined for 26.Four years ago, with Russia competing on home turf and in the midst of what investigators have determined was a state-sponsored doping conspiracy, they looked like this:“Maybe the only good byproduct of the past corruption and scandal is that it forces sport to make sure it never happens again,” said Travis Tygart, the CEO of the U.S. Anti-Doping Agency.The 19 competed as neutral athletes because the IAAF — track and field’s world governing body — determined they’ve been under proper anti-doping controls.Jamaica has also been under increased scrutiny, which has led to the stripping of a 2008 Olympic relay medal because of doping by one of Usain Bolt’s teammates.The IAAF has established an independent testing authority that takes responsibilities for conducting the antidoping program out of its hands.“I thought in 2012, I was in a cleaner race, and it turns out that Olympic final was probably one of the dirtiest in history,” said San Francisco middle-distance runner Shannon Rowbury, who finished out of the medals in a race in which no fewer than five of the 1,500-meter finalists have been investigated for doping violations.